772 research outputs found
Stellar ArAr reactions and their effect on light neutron-rich nuclide synthesis
The ArAr ( = 35 d) and
ArAr (269 y) reactions were studied for the first time
with a quasi-Maxwellian ( keV) neutron flux for Maxwellian Average
Cross Section (MACS) measurements at stellar energies. Gas samples were
irradiated at the high-intensity Soreq applied research accelerator
facility-liquid-lithium target neutron source and the Ar/Ar and
Ar/Ar ratios in the activated samples were determined by
accelerator mass spectrometry at the ATLAS facility (Argonne National
Laboratory). The Ar activity was also measured by low-level counting at
the University of Bern. Experimental MACS of Ar and Ar, corrected
to the standard 30 keV thermal energy, are 1.9(3) mb and 1.3(2) mb,
respectively, differing from the theoretical and evaluated values published to
date by up to an order of magnitude. The neutron capture cross sections of
Ar are relevant to the stellar nucleosynthesis of light neutron-rich
nuclides; the two experimental values are shown to affect the calculated mass
fraction of nuclides in the region A=36-48 during the weak -process. The new
production cross sections have implications also for the use of Ar and
Ar as environmental tracers in the atmosphere and hydrosphere.Comment: 18 pages + Supp. Mat. (13 pages) Accepted for publication in Phys.
Rev. Let
RAD sequencing resolves fine-scale population structure in a benthic invertebrate: implications for understanding phenotypic plasticity.
The field of molecular ecology is transitioning from the use of small panels of classical genetic markers such as microsatellites to much larger panels of single nucleotide polymorphisms (SNPs) generated by approaches like RAD sequencing. However, few empirical studies have directly compared the ability of these methods to resolve population structure. This could have implications for understanding phenotypic plasticity, as many previous studies of natural populations may have lacked the power to detect genetic differences, especially over micro-geographic scales. We therefore compared the ability of microsatellites and RAD sequencing to resolve fine-scale population structure in a commercially important benthic invertebrate by genotyping great scallops (Pecten maximus) from nine populations around Northern Ireland at 13 microsatellites and 10 539 SNPs. The shells were then subjected to morphometric and colour analysis in order to compare patterns of phenotypic and genetic variation. We found that RAD sequencing was superior at resolving population structure, yielding higher Fst values and support for two distinct genetic clusters, whereas only one cluster could be detected in a Bayesian analysis of the microsatellite dataset. Furthermore, appreciable phenotypic variation was observed in size-independent shell shape and coloration, including among localities that could not be distinguished from one another genetically, providing support for the notion that these traits are phenotypically plastic. Taken together, our results suggest that RAD sequencing is a powerful approach for studying population structure and phenotypic plasticity in natural populations
Lectures on F-theory compactifications and model building
These lecture notes are devoted to formal and phenomenological aspects of
F-theory. We begin with a pedagogical introduction to the general concepts of
F-theory, covering classic topics such as the connection to Type IIB
orientifolds, the geometry of elliptic fibrations and the emergence of gauge
groups, matter and Yukawa couplings. As a suitable framework for the
construction of compact F-theory vacua we describe a special class of
Weierstrass models called Tate models, whose local properties are captured by
the spectral cover construction. Armed with this technology we proceed with a
survey of F-theory GUT models, aiming at an overview of basic conceptual and
phenomenological aspects, in particular in connection with GUT breaking via
hypercharge flux.Comment: Invited contribution to the proceedings of the CERN Winter School on
Supergravity, Strings and Gauge Theory 2010, to appear in Classical and
Quantum Gravity; 63 pages; v2: references added, typos correcte
One in a Billion: MSSM-like D-Brane Statistics
Continuing our recent work hep-th/0411173, we study the statistics of
four-dimensional, supersymmetric intersecting D-brane models in a toroidal
orientifold background. We have performed a vast computer survey of solutions
to the stringy consistency conditions and present their statistical
implications with special emphasis on the frequency of Standard Model features.
Among the topics we discuss are the implications of the K-theory constraints,
statistical correlations among physical quantities and an investigation of the
various statistical suppression factors arising once certain Standard Model
features are required. We estimate the frequency of an MSSM like gauge group
with three generations to be one in a billion.Comment: 36 pages, 12 figures; v2: typos corrected, one ref. added; v3: minor
changes, version to appear in JHE
Polarisation selective magnetic vortex dynamics and core reversal in rotating magnetic fields
A magnetic vortex occurs as an equilibrium configuration in thin
ferromagnetic platelets of micron and sub-micron size and is characterised by
an in-plane curling magnetisation. At the centre, a magnetic singularity is
avoided by an out-of-plane magnetisation core. This core has a gyrotropic
excitation mode, which corresponds to a circular motion of the vortex around
its equilibrium position, where the rotation sense is determined by the
direction of the vortex core magnetisation, its polarisation. Unlike linear
fields or spin polarised currents, which excite both polarisation states, an
in-plane rotating field can selectively excite one of the polarisation states.
Here we report the observation of vortex dynamics in response to rotating
magnetic fields, imaged with time-resolved scanning X-ray microscopy. We
demonstrate that the rotating field only excites the gyrotropic mode if the
rotation sense of the field coincides with the vortex gyration sense and that
such a field can selectively reverse the vortex polarisation
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